This study was designed to answer the question: Is H3-thymidine uptake by nuclei of the mouse seminal vesicle evidence for DNA synthesis and mitosis, or does it signify some "metabolic" function of DNA unrelated to chromosome duplication?

Mice were given an intraperitoneal injection of H3-thymidine. Six hours later Feulgen squashes of the seminal vesicle epithelium were made and covered with autoradiographic stripping film. The silver grains above labeled nuclei were counted, and the Feulgen dye contents of these same nuclei were determined photometrically after removal of the grains from the emulsion. Unlabeled nuclei were also measured.

The dye contents of non-radioactive nuclei form a unimodal distribution, indicating that polyploidy is absent from this tissue. The radioactive nuclei fall into two groups. In the first, the average dye content is the same as that of the cold nuclei (2C). In the second, the values range from 2C to 4C.

In the 2C to 4C group the grain count is proportional to the dye content, showing that incorporation is correlated with synthesis.

The radioactive 2C nuclei arose by mitosis during the course of the experiment. This is shown by the following facts: (1) They frequently occur in pairs. (2) They average smaller than unlabeled 2C nuclei. (3) Their average grain count is approximately half that of the 4C nuclei. (4) Labeled division figures are found. (5) A mitotic rate estimated from the number of labeled 2C nuclei accords reasonably well with one based on the number of observed mitoses.

Since the incorporation of thymidine accompanies DNA synthesis and precedes mitosis, there is no reason to postulate a special "metabolic" DNA in this tissue.

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